Ct detector having a segmented optical coupler and method of manufacturing same

ABSTRACT

The present invention is a directed to a CT detector for a CT imaging system that incorporates a segmented optical coupler between a photodiode array and a scintillator array. The segmented optical coupler also operates as a light collimator which improves the light collection efficiency of the photodiode array. The segmented optical coupler is defined by a series of reflector elements that collectively form a plurality of open cells. The open cells form light transmission cavities and facilitate the collimation of light from a scintillator to a photodiode. The cavities may be filled with optical epoxy for sealing to the photodiode array.

BACKGROUND OF INVENTION

[0001] The present invention relates generally to diagnostic imagingand, more particularly, to a CT detector having a segmented ornon-contiguous optical coupler and method of manufacturing same.Additionally, the segmented optical coupler operates as a lightcollimator integrally formed between the scintillators and photodiodesof the detector.

[0002] Typically, in computed tomography (CT) imaging systems, an x-raysource emits a fan-shaped beam toward a subject or object, such as apatient or a piece of luggage.

[0003] Hereinafter, the terms “subject” and “object” shall includeanything capable of being imaged. The beam, after being attenuated bythe subject, impinges upon an array of radiation detectors. Theintensity of the attenuated beam radiation received at the detectorarray is typically dependent upon the attenuation of the x-ray beam bythe subject. Each detector element of the detector array produces aseparate electrical signal indicative of the attenuated beam received byeach detector element. The electrical signals are transmitted to a dataprocessing system for analysis which ultimately produces an image.

[0004] Generally, the x-ray source and the detector array are rotatedabout the gantry within an imaging plane and around the subject. X-raysources typically include x-ray tubes, which emit the x-ray beam at afocal point. X-ray detectors typically include a collimator forcollimating x-ray beams received at the detector, a scintillator forconverting x-rays to light energy adjacent the collimator, andphotodiodes for receiving the light energy from the adjacentscintillator and producing electrical signals therefrom.

[0005] Typically, each scintillator of a scintillator array convertsx-rays to light energy. Each scintillator discharges light energy to aphotodiode adjacent thereto. Each photodiode detects the light energyand generates a corresponding electrical signal. The outputs of thephotodiodes are then transmitted to the data processing system for imagereconstruction.

[0006] “Cross talk” between detector cells of a CT detector is common.“Cross talk” is generally defined as the communication of data betweenadjacent cells of a CT detector. Generally, cross talk is sought to bereduced as cross talk leads to artifact presence in the finalreconstructed CT image and contributes to poor spatial resolution.Typically, four different types of cross talk may result within a singleCT detector. X-ray cross talk may occur due to x-ray scattering betweenscintillator cells. Optical cross talk may occur through thetransmission of light through the reflectors that surround thescintillators. Known CT detectors utilize a contiguous optical couplinglayer(s), typically epoxy, to secure the scintillator array to thephotodiode array. Cross talk, however, can occur as light from one cellis passed to another through the contiguous layer. Electrical cross talkcan occur from unwanted communication between photodiodes. Of the abovetypes of cross talk, cross talk though the contiguous optical couplerlayer(s) is generally considered a major source of cross talk in the CTdetector.

[0007] Therefore, it would be desirable to design a CT detector havingimproved optical coupling between the scintillator array and photodiodearray to reduce cross talk in the CT detector and improve spatialresolution of the final reconstructed image.

BRIEF DESCRIPTION OF THE INVENTION

[0008] The present invention is a directed to a CT detector for a CTimaging system that overcomes the aforementioned drawbacks. The CTdetector incorporates a gridded light collimator between a photodiodearray and a scintillator array. The light collimator improves the lightcollection efficiency of the photodiode array and may be formed ofreflector material so as to reduce cross talk within the detector. Eachgridded collimator is defined by a series of reflector elements thatcollectively form a plurality of open cells. The open cells form lighttransmission cavities and facilitate the collimation of light from ascintillator to a photodiode. The cavities may be filled with opticalepoxy for sealing to the photodiode array or scintillator array therebyavoiding the drawbacks associated with contiguous optical couplerlayers.

[0009] Therefore, in accordance with the present invention, a CTdetector includes a plurality of scintillators arranged in an array toreceive x-rays and output light in response to the received x-rays. Aplurality of light detection elements are arranged in an arraydimensionally similar to the scintillator array and are configured todetect light from the scintillators. A non-contiguous optical coupler isthen used to secure the plurality of scintillators to the plurality oflight detection elements.

[0010] According to another aspect of the present invention, a CT systemincludes a rotatable gantry having a bore centrally disposed therein anda table movable fore and aft through the bore and configured to positiona subject for CT data acquisition. A high frequency electromagneticenergy projection source is positioned within the rotatable gantry andconfigured to project high frequency electromagnetic energy toward thesubject. The CT system further includes a detector array disposed withinthe rotatable gantry and configured to detect high frequencyelectromagnetic energy projected by the projection source and impingedby the subject. The detector array includes a plurality of scintillatorsarranged in a scintillator array as well as a plurality of photodiodesarranged in a photodiode array. A light collimator having a plurality oflight transmission cavities is disposed between the scintillator arrayand the photodiode array.

[0011] In accordance with a further aspect of the present invention, amethod of CT detector manufacturing includes the steps of forming ascintillator array having a plurality of scintillators and forming aphotodiode array having a plurality of photodiodes. An open-celledcollimator is then deposited between the arrays. The resulting assemblyis then secured to one another.

[0012] Various other features, objects and advantages of the presentinvention will be made apparent from the following detailed descriptionand the drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0013] The drawings illustrate one preferred embodiment presentlycontemplated for carrying out the invention.

[0014] In the drawings:

[0015]FIG. 1 is a pictorial view of a CT imaging system.

[0016]FIG. 2 is a block schematic diagram of the system illustrated inFIG. 1.

[0017]FIG. 3 is a perspective view of one embodiment of a CT systemdetector array.

[0018]FIG. 4 is a perspective view of one embodiment of a detector.

[0019]FIG. 5 is illustrative of various configurations of the detectorin FIG. 4 in a four-slice mode.

[0020]FIG. 6 is a schematic of a cross-section of a CT detector inaccordance with the present invention.

[0021]FIGS. 7-10 set forth steps of various techniques of manufacturinga CT detector in accordance with the present invention.

[0022]FIG. 11 is a pictorial view of a CT system for use with anon-invasive package inspection system.

DETAILED DESCRIPTION

[0023] The operating environment of the present invention is describedwith respect to a four-slice computed tomography (CT) system. However,it will be appreciated by those skilled in the art that the presentinvention is equally applicable for use with single-slice or othermulti-slice configurations. Moreover, the present invention will bedescribed with respect to the detection and conversion of x-rays.However, one skilled in the art will further appreciate that the presentinvention is equally applicable for the detection and conversion ofother high frequency electromagnetic energy. The present invention willbe described with respect to a “third generation” CT scanner, but isequally applicable with other CT systems.

[0024] Referring to FIGS. 1 and 2, a computed tomography (CT) imagingsystem 10 is shown as including a gantry 12 representative of a “thirdgeneration” CT scanner. Gantry 12 has an x-ray source 14 that projects abeam of x-rays 16 toward a detector array 18 on the opposite side of thegantry 12. Detector array 18 is formed by a plurality of detectors 20which together sense the projected x-rays that pass through a medicalpatient 22. Each detector 20 produces an electrical signal thatrepresents the intensity of an impinging x-ray beam and hence theattenuated beam as it passes through the patient 22. During a scan toacquire x-ray projection data, gantry 12 and the components mountedthereon rotate about a center of rotation 24.

[0025] Rotation of gantry 12 and the operation of x-ray source 14 aregoverned by a control mechanism 26 of CT system 10. Control mechanism 26includes an x-ray controller 28 that provides power and timing signalsto an x-ray source 14 and a gantry motor controller 30 that controls therotational speed and position of gantry 12. A data acquisition system(DAS) 32 in control mechanism 26 samples analog data from detectors 20and converts the data to digital signals for subsequent processing. Animage reconstructor 34 receives sampled and digitized x-ray data fromDAS 32 and performs high speed reconstruction. The reconstructed imageis applied as an input to a computer 36 which stores the image in a massstorage device 38.

[0026] Computer 36 also receives commands and scanning parameters froman operator via console 40 that has a keyboard. An associated cathoderay tube display 42 allows the operator to observe the reconstructedimage and other data from computer 36. The operator supplied commandsand parameters are used by computer 36 to provide control signals andinformation to DAS 32, x-ray controller 28 and gantry motor controller30. In addition, computer 36 operates a table motor controller 44 whichcontrols a motorized table 46 to position patient 22 and gantry 12.Particularly, table 46 moves portions of patient 22 through a gantryopening 48.

[0027] As shown in FIGS. 3 and 4, detector array 18 includes a pluralityof scintillators 57 forming a scintillator array 56. A collimator (notshown) is positioned above scintillator array 56 to collimate x-raybeams 16 before such beams impinge upon scintillator array 56.

[0028] In one embodiment, shown in FIG. 3, detector array 18 includes 57detectors 20, each detector 20 having an array size of 16×16. As aresult, array 18 has 16 rows and 912 columns (16×57 detectors) whichallows 16 simultaneous slices of data to be collected with each rotationof gantry 12.

[0029] Switch arrays 80 and 82, FIG. 4, are multi-dimensionalsemiconductor arrays coupled between scintillator array 56 and DAS 32.Switch arrays 80 and 82 include a plurality of field effect transistors(FET) (not shown) arranged as multi-dimensional array. The FET arrayincludes a number of electrical leads connected to each of therespective photodiodes 60 and a number of output leads electricallyconnected to DAS 32 via a flexible electrical interface 84.Particularly, about one-half of photodiode outputs are electricallyconnected to switch 80 with the other one-half of photodiode outputselectrically connected to switch 82. Additionally, a reflector layer(not shown) may be interposed between each scintillator 57 to reducelight scattering from adjacent scintillators. Each detector 20 issecured to a detector frame 77, FIG. 3, by mounting brackets 79.

[0030] Switch arrays 80 and 82 further include a decoder (not shown)that enables, disables, or combines photodiode outputs in accordancewith a desired number of slices and slice resolutions for each slice.Decoder, in one embodiment, is a decoder chip or a FET controller asknown in the art. Decoder includes a plurality of output and controllines coupled to switch arrays 80 and 82 and DAS 32. In one embodimentdefined as a 16 slice mode, decoder enables switch arrays 80 and 82 sothat all rows of the photodiode array 52 are activated, resulting in 16simultaneous slices of data for processing by DAS 32. Of course, manyother slice combinations are possible. For example, decoder may alsoselect from other slice modes, including one, two, and four-slice modes.

[0031] As shown in FIG. 5, by transmitting the appropriate decoderinstructions, switch arrays 80 and 82 can be configured in thefour-slice mode so that the data is collected from four slices of one ormore rows of photodiode array 52. Depending upon the specificconfiguration of switch arrays 80 and 82, various combinations ofphotodiodes 60 can be enabled, disabled, or combined so that the slicethickness may consist of one, two, three, or four rows of scintillatorarray elements 57. Additional examples include, a single slice modeincluding one slice with slices ranging from 1.25 mm thick to 20 mmthick, and a two slice mode including two slices with slices rangingfrom 1.25 mm thick to 10 mm thick. Additional modes beyond thosedescribed are contemplated.

[0032] Referring now to FIG. 6, a schematic of a cross-section of a CTdetector 20 is shown. As discussed above, detector 20 includes ascintillator array 56 defined by a plurality of scintillators 57. Eachof the scintillators is designed to generate a light output 85 inresponse the reception of x-rays 16. A reflector layer 86 coats thex-ray reception surface of the scintillators to improve light collectionefficiency of the photodiodes. The reflector layer 86 is composed of amaterial that allows x-rays projected from a projection source to passthrough and reflects light generated by the scintillators back towardthe photodiodes. The reflector layer is integrated with a series ofreflector elements 88 that extend between adjacent scintillators 57 as areflector wall. The reflector elements 88 are designed to prevent lightscattering and/or reduce x-ray scattering between scintillators.

[0033] CT detector 20 is constructed such that a light cavity 90 extendsbetween each photodiode and scintillator. The light cavity may beconstructed in accordance with a number of fabrication techniques aswill be described with respect to FIGS. 7-10 and is defined by cavityelements or plates 92. Plates 92 are preferably formed of a reflectormaterial similar to that used to form reflector elements 88.Additionally, plates 92 have a width similar to the width of thereflector elements 88. Preferably, plates 92 are formed during theformation of reflector elements 88, as will be described with respect toFIG. 7. As such, plates 92 extend from reflector elements to the lightdetection surface of the photodiode array.

[0034] Plates 92 are constructed to form light transmission cavities 90and, as such, operate as an inner-cell light collimator. Plates 92 aredesigned to eliminate light cross talk between scintillators therebycollimating light toward the light detection surfaces of the photodiodearray. Further, plates 92 may be coated with an optical coupling film orresin so as to secure the plates to the photodiode array. Alternately,the plates may be bonded to the surface of the photodiode array. In afurther embodiment, each of the light transmission cavities 90 is filledwith an optical epoxy similar to the epoxy used in a contiguous epoxylayer. The optical epoxy operates as adhesive to connect the photodiodearray to the scintillator array. With the presence of reflector plates92, the drawbacks associated with contiguous optical layer cross talkare avoided. While epoxy may be used to secure the arrays to oneanother, other composites and materials such as thermoplastics may beused and are within the scope of the invention.

[0035] Referring now to FIG. 7, steps for a technique of manufacturing aCT detector similar to that described with respect to FIG. 6 are shown.The steps illustrated may be carried out by a labor intensive process, afully automated, computer driven process, or a combination thereof.Technique 100 begins at 102 with the assimilation of products, personneland the like for CT detector fabrication. That achieved during this stepmay vary but, at a minimum, should include the preparation of ascintillator block. The scintillator block is then mounted onto adissolvable material 104. The scintillator block and dissolvablematerial are then diced or cut at 106. Once cut, either along one or twodimensions, a plurality of scintillator cells uniformly spaced from oneanother results. Reflector material is then cast at 108 in the voidscreated between the scintillator cells as a result of the dicingprocess. The reflector material should be cast such that the interfacebetween scintillators is completely filled as is the interface betweenadjacent portions of the dissolvable material. The cast reflectormaterial is then allowed to cure and undergoes any additional processingto insure proper reflectivity and the like. Once the cast reflectormaterial has cured, the dissolvable material is dissolved at 110. Theprocess for dissolving the material depends on the type of dissolvablematerial used. For example, the dissolvable material may be placed in awash and chemically dissolved or heated at a specified temperature to,in essence, “melt” away the dissolvable material. After the dissolvingprocess is complete, a scintillator array with an integrated castreflector results. Of particular note is that each reflector elementbetween the scintillators extends beyond the scintillator, i.e. has agreater length than the scintillators. The portion of the reflector thatextends beyond the scintillator operates as a reflector plate asdescribed above. The open cells that result between reflector platesdefine a light transmission cavity and are filled with optical epoxy at112. The optical epoxy permits the transmission of light betweenscintillator and photodiode while simultaneously creating an adhesioninterface for coupling the scintillator to the photodiode. As such, thephotodiode array and scintillator array are coupled to one another at114. This portion of the CT detector fabrication process is thencomplete and the remainder of the CT detector fabricating takes placedownstream at 116.

[0036] The CT detector described with respect to FIG. 6 and fabricatedaccording to the technique of FIG. 7 illustrates only one example of thepresent invention. A similar CT detector incorporating the advantages ofthat described with respect to FIG. 6 and fabricated in accordance withtechniques different from that illustrated in FIG. 7 are contemplatedand within the scope of this invention. For purpose of illustration andnot limitation, additional manufacturing techniques and the resultingstructures will be described with reference to FIGS. 8-10.

[0037] Referring now to FIG. 8, another CT manufacturing process 118begins at 120 with a block of scintillator material being prepared. Theblock is then placed onto a block of thermoplastic material at 122. Thescintillator block and the thermoplastic are then diced or cut 124 inaccordance with known dicing processes. Preferably, only a portion ofthe thermoplastic is diced thereby leaving a thin, uncut portion thatcan be used to seal against the photodiode array. Cast reflector is thendeposited in the voids 126 between scintillator cells that result fromthe dicing process. In contrast to the CT detector constructed inaccordance with FIG. 7, an optical epoxy between the reflector platesformed by the cast reflector is not used. Because the thermoplasticmaterial is not completely diced through, a thin thermoplastic layerresults that, as discussed above, is used to secure the scintillatorarray to the photodiode array as opposed to an optical epoxy. Process118 then concludes at 128 with the CT detector undergoing additionalprocessing and fabrication in accordance with known techniques.

[0038] The processes described above involve alternations to thescintillator array. In contrast, the process of FIG. 9 creates thereflector plates by etching the photodiode array. Specifically, process130 begins at 132 with the formation of a photodiode array. At 134, thephotodiode array is coated with a film of semiconductor or othersuitable materials. Preferably, a thin layer of Silicon is applied orthermally grown and allowed to cure to the photodiode light receptionsurface. Semiconductor materials that will not adversely affect thelight collection abilities of the photodiode array should be used. Thesurface of the photodiode array is then masked and plasma etched at 136using standard semiconductor fabrication techniques to form a grid.Various semiconductor fabrication processes are contemplated includingchemical etching, mechanical etching, ion beam milling, and the like.The result of the etching process should result in a series of opencells defined by the semiconductor material. The open cells should bevertically aligned with the light detection surfaces of the photodiodearray. The open cells are then filled with optical epoxy 138 to securethe photodiode array to the scintillator array at 140. The resultingassembly then undergoes standard post-processing techniques whereuponthe process ends at 142.

[0039] The process illustrated in FIG. 10 utilizes an intermediaryelement that is not integrated with the scintillator array or photodiodearray. Manufacturing process 144 begins at 146 with the formation of ascintillator array and a photodiode array in accordance with knownfabrication techniques. A grid is then etched at 148 from a sheet ofthin metallic or other material. The grid defines a number of cellsdimensionally equivalent to the scintillators and photodiodes.Additionally, the grid preferably has a height equal to the desiredheight of the light transmission cavities heretofore described.Accordingly, the open cells formed in the grid are aligned with theeither the scintillators of the scintillator array or with thephotodiodes of the photodiode array at 150. The grid is then bonded at152 to the selected array. The open cells or cavities defined by thegrid may then be filled with optical epoxy at 154. The optical epoxy isthen used to secure the selected array to the other array at 156.Alternately, the open cells may be left empty and the grid bonded to theother array. The process is then complete at 158.

[0040] Each of the above-described manufacturing processes results in aCT detector having a non-contiguous optical coupler thereby avoiding thedrawbacks associated with a contiguous optical coupler layer. Each ofthe processes produces a CT detector wherein a light transmission cavityis formed to collimate light emissions from a scintillator to aphotodiode. The cavity may be filled with optical coupling epoxy or leftempty and the scintillator bonded to the photodiode array. It ispreferred that the cavities be filled with epoxy as this results inbetter optical transmission and a stronger connection being formedbetween the scintillator and photodiode.

[0041] Referring now to FIG. 11, package/baggage inspection system 160includes a rotatable gantry 162 having an opening 164 therein throughwhich packages or pieces of baggage may pass. The rotatable gantry 162houses a high frequency electromagnetic energy source 166 as well as adetector assembly 168. A conveyor system 170 is also provided andincludes a conveyor belt 172 supported by structure 174 to automaticallyand continuously pass packages or baggage pieces 176 through opening 164to be scanned. Objects 176 are fed through opening 164 by conveyor belt172, imaging data is then acquired, and the conveyor belt 172 removesthe packages 176 from opening 164 in a controlled and continuous manner.As a result, postal inspectors, baggage handlers, and other securitypersonnel may non-invasively inspect the contents of packages 176 forexplosives, knives, guns, contraband, etc.

[0042] Therefore, in accordance with one embodiment of the presentinvention, a CT detector includes a plurality of scintillators arrangedin an array to receive x-rays and output light in response to thereceived x-rays. A plurality of light detection elements are arranged inan array dimensionally similar to the scintillator array and areconfigured to detect light from the scintillators. A non-contiguousoptical coupler is then used to secure the plurality of scintillators tothe plurality of light detection elements.

[0043] According to another embodiment of the present invention, a CTsystem includes a rotatable gantry having a bore centrally disposedtherein and a table movable fore and aft through the bore and configuredto position a subject for CT data acquisition. A high frequencyelectromagnetic energy projection source is positioned within therotatable gantry and configured to project high frequencyelectromagnetic energy toward the subject. The CT system furtherincludes a detector array disposed within the rotatable gantry andconfigured to detect high frequency electromagnetic energy projected bythe projection source and impinged by the subject. The detector arrayincludes a plurality of scintillators arranged in a scintillator arrayas well as a plurality of photodiodes arranged in a photodiode array. Alight collimator having a plurality of light transmission cavities isdisposed between the scintillator array and the photodiode array.

[0044] In accordance with a further embodiment of the present invention,a method of CT detector manufacturing includes the steps of forming ascintillator array having a plurality of scintillators and forming aphotodiode array having a plurality of photodiodes. An open-celledcollimator is then deposited between the arrays. The resulting assemblyis then secured to one another.

[0045] The present invention has been described in terms of thepreferred embodiment, and it is recognized that equivalents,alternatives, and modifications, aside from those expressly stated, arepossible and within the scope of the appending claims.

What is claimed is:
 1. A CT detector comprising: a plurality ofscintillators arranged in an array to receive x-rays and output light inresponse to the reception of x-rays; a plurality of light detectionelements arranged in an array to output electrical signals in responseto light detected from the plurality of scintillators; and anon-contiguous optical coupler to secure the plurality of scintillatorsto the plurality of light detection elements.
 2. The CT detector ofclaim 1 wherein the non-contiguous optical coupler includes a pluralityof integrated reflector elements.
 3. The CT detector of claim 2 whereinthe plurality of integrated reflector elements are arranged to define aplurality of optical coupling cells, each cell configured to secure onescintillation to one photodiode.
 4. The CT detector of claim 3 whereineach optical coupling cell includes an optical epoxy.
 5. The CT detectorof claim 3 wherein each optical coupling cell includes a thermoplasticmaterial.
 6. The CT detector of claim 1 wherein the non-contiguousoptical coupler includes a gridded screen defining a plurality of lighttransmission cavities between the plurality of scintillators and theplurality of light detection elements.
 7. The CT detector of claim 6wherein the gridded screen includes an etched metallic grid bonded tothe plurality of scintillators.
 8. The CT detector of claim 6 whereinthe light transmission cavities include optical coupling epoxy.
 9. A CTsystem comprising: a rotatable gantry having a bore centrally disposedtherein; a table movable fore and aft through the bore and configured toposition a subject for CT data acquisition; a high frequencyelectromagnetic energy projection source positioned within the rotatablegantry and configured to project high frequency electromagnetic energytoward the subject; and a detector array disposed within the rotatablegantry and configured to detect high frequency electromagnetic energyprojected by the projection source and impinged by the subject, thedetector array including: a plurality of scintillators arranged in ascintillator array; a plurality of photodiodes arranged in a photodiodearray; and a light collimator having a plurality of light transmissioncavities disposed between the plurality of scintillators and theplurality of photodiodes.
 10. The CT system of claim 9 wherein theplurality of light transmission cavities is filled with an opticalepoxy.
 11. The CT system of claim 9 wherein the plurality of lighttransmission cavities is filled with a thermoplastic material.
 12. TheCT system of claim 9 wherein the detector array includes a thin filmoptical coupler coupling the light collimator to the plurality ofphotodiodes.
 13. The CT system of claim 9 wherein each lighttransmission cavity is defined by a number of reflector elements. 14.The CT system of claim 13 wherein each light transmission cavitycollimates light between a single scintillator and a single photodiode.15. A method of manufacturing a CT detector comprising the steps of:forming a scintillator array having a plurality of scintillators;forming a photodiode array having a plurality of photodiodes; depositingan open-celled collimator between the scintillator array and thephotodiode array; and securing the scintillator, the open-celledcollimator, and the photodiode array to one another.
 16. The method ofclaim 15 wherein the steps of forming the scintillator array anddepositing an open-celled collimator include the steps of: mounting ablock of scintillator material onto a dissolvable substrate; dicingthrough the block and the dissolvable substrate to form a plurality ofvoids; casting reflector material into the plurality of voids; anddissolving the dissolvable substrate to form the open-celled collimator.17. The method of claim 16 further comprising the step of filling eachcell of the open-celled collimator with an optical coupling substance.18. The method of claim 17 wherein the optical coupling substanceincludes epoxy.
 19. The method of claim 16 wherein the step of securingthe scintillator, the open-celled collimator, and the photodiode arrayincludes the step of coupling a thin layer of adhesive to theopen-celled collimator.
 20. The method of claim 15 wherein the steps offorming the scintillator array and depositing the open-celled collimatorinclude the steps of: mounting a block of scintillator material onto athermoplastic substrate; dicing through the block of scintillatormaterial and at least partially through the thermoplastic material toform a plurality of voids; and casting reflector material into theplurality of voids.
 21. The method of claim 15 wherein the step ofdepositing an open-celled collimator includes the step of depositing athermoplastic material with an embedded mesh between the scintillatorarray and the photodiode array, the mesh defining a plurality of lighttransmission cavities.
 22. The method of claim 21 further comprising thestep of filling the plurality of light transmission cavities with anoptical epoxy.
 23. The method of claim 15 wherein the step of depositingan open-celled collimator further includes the steps of: coating theplurality of photodiodes with a semiconductor material; and etching thesemiconductor material to form a plurality of light transmissioncavities.
 24. The method of claim 23 further comprising the step offilling the plurality of light transmission cavities with optical epoxy.25. The method of claim 15 wherein the open-celled collimator includes ametallic grid having a plurality of light transmission cavities.
 26. Themethod of claim 25 further comprising the step of etching the metallicgrid from a substrate.
 27. The method of claim 25 wherein the step ofsecuring includes the step of bonding the metallic grid to the pluralityof scintillators and the plurality of photodiodes.
 28. The method ofclaim 25 further comprising the step of filling the plurality of lighttransmission cavities with optical epoxy.